The problem was traced to water in the coax, it entered at the antenna connection. The antenna has tested out ok so we have refitted it at a reduced height on a temporary coax. New coax will be ordered and installed when time and weather permits. The repeater is back on air with a reduced antenna height so we expect a reduced coverage area.
There appears to be a transmitter/receiver problem with GB3RF this evening so it's been switched off to protect the equipment until it can be checked over tomorrow. Will update when more information is available. Update - 03/12/2018 After checking the repeater today we've had to report it as not operational to the ETCC because we don't know how long repairs will take.